How to get CONNECTED to the fuel pressure line to do tests?
All these cars (Sidekick, Tracker, Vitara, later Samurai ) have a banjo test port, that works with any Harbor Fright(pun) fuel gauge kit.

The fuel filter banjo, is fitted with a M6 (6mm) screw and the HF kit has a Suzuki hose that fit there.
If you have the MPI engine, the fuel rail can be fitting with a better test port. per below. (no crawling under car , over and over doing the injector leak down tests)
All fittings added or used must be, 60 PSI or more rated, never us CARB fuel lines, or clamps.
Do not use car tire (AIR) schrader cores in any HVAC or Fuel system.
You can also buy this kit that has all the fittings.

GOOD  HOSE CLAMPS  and bad clamps.
The left is FI rated clamp the right side clamp is not. 
The Right side clamp is for air/water/carb cars.  The smaller the hose, the worse the right clamp works correctly.
Carb cars are 15 PSI cars, near zip compared to Fuel injection.
The force it applies is not equal , all around the band.
Best is to match your hose size exactly to the left clamp here. The difficulty, is the hoses have different O.D. and wall thicknesses.
It must be safe at 60+ PSI. (minimum !)
I mention this because many Samurai cars, were converted to Fuel injection, and done unsafely.
Best practice, on a conversion is to put the "T" fitting seen here  in-line near tank and then put NPT schrader valve (pressures side) on that "T".
Then cap it off that "T" with pipe fitting plug. (so at any time , you can fit up a fuel gauge)

HOW TO , install a Schrader valve or like adaptor to the MPI front end of Fuel rail or to an adaptor for same:

          I think this one, is Corvette type.                                 
Must be fuel rated , not AIR or tire , rated. ! 
Many guys use "Viton Schrader valves in Refrigeration fittings". HVAC rated.
There are many ways to get a fuel pressure reading on a 16valve engine 91-98'.  
 (8v motor you must use the filter banjo test port)  on 16v you can use both ports,  rail or filter)

Easier to find, is this Ford Schrader valve , brazed to the below summit adaptor or tapped directly into you rail or rail cap bolt.
Motorcraft CM3461  

The 16V ONLY cars can use this nice low cost  Fuel Rail adaptor:
For the replacement of the the front fuel RAIL plug !
ATM-2267  Summit , @ $10
Summit Racing is a good source
Summit also lists this part on  ATM-2267 
You can use the AT-2267 directly to you gage. A 1/8" NPT pipe thread it is.  Easy to adapt anything in this form.

On my 1997 Tracker,  I had I installed a (ProSport Gauge Electric sender, screwed into the above fitting, directly.)

I found this store, has lots more adaptors, than most.
EGAUGES  store

I connected my  fuel Gauge sender, directly to ATM2267 above, for my  electric fuel guage.
end ATM2267 mod's.

The Ford Schrader mod.
The following V8 engine Ford EFI fuel aux port valve,  can be fitted to the rail plug , part ID #3

The P/N is ; Motorcraft CM3461  
and is sold everywhere and at Amazon.  $9

and  $5

Orielly's has it. use the Motorcraft #

One could remove the special Schrader core (bicycle tire core tool) and braze the CM3461 to the  ATM-2267.
Do not use bicycle cores, use HVAC rated cores.  (Viton) with color coded ring seals to identify this fact.
The correct cores to use are HVAC cores.
If you weld on the Schrader valve , un-screw the valve internal part so it don't overheat or melt the seals.
Here is the Ranger88 Schrader kit.
The tap  Ranger  used was a 1/16 - 27 NPTF and was recommended that I use a reamer prior to tap. Assembled with Permatex thread sealant.
Ford Aux fuel rail fitting (schrader) to Stock Suzuki fuel rail plug , modifaction.  

end  stock rail plug to schrader adaption:

Suzuki 16valve  MPI Rail set up:  92-98 .

Suzuki P/N for ID 3 is:;    09247- 14029  (92-98 16v)    (drill it and make it up, any way you want)
Suzuki P/N for ID 13 is:   09168- 14012    "
Part 13 washers, are fussy, I found most non suzuki washers used here, to leak.

DIY: The  6mm bolt solution:  This makes part 3 above, match the fuel filter banjo bolt 6mm test port.
Remove the part #3  (I bought spare) about and drill it out to the tap size 6mm x 1.0P   use a  5.2mm drill as the tap drill.
1.0P = 1.0 PITCH.   
A USA SAE number drill # 6, =  0.204"-- (5.18mm) will work too,  these 2 tools, a drill bit and tap are very low cost. 
#6 and 6mm have no relation ship , it is coincidence.
After drilling ,tapping and testing just plug the hole with any 6mmx1P screw and copper washer 6mm.  Ready for the next test.
This allows the 6mm Harbor freight tool to fit this rail fitting a better idea might be to put a shrader valve fitted to part 3 above.
end 6mm mod.

Source of fittings for fuel rails , adapters, and gauges, etc.:

Harbor Fright (Freight) has this and the right hose fits all 89-95 fuel filter banjo bolts (6mm built in test port)

A much better gage but you need to adapt it. From OTC.

Harbor freight $20 gage.
 (not the best quality but the hose is nice , it fits all Suzuki's  89-98  8v or 16v comes with Suzuki hose.
Below is a 91 Tracker. But all 89-98' use this setup.

Came with my Fuel gage.  Schrader to  "AN" fiitting .
This adaptor allows usage on your existing fuel gauge  hose.  I use this on all cars with Schrader fuel test ports.

The 8V  TBI fuel test adapter is Suzuki P/N  J37746 -- fits on fuel filter   (89-98)   Don't ask price, In$ane !
The 16v MPI fuel test adapter is Suzuki P/N  J41041 -- fits fuel rail and   matches Gauge set J34730-b

See  (SKU: 09919-46010-B)   and the one same pn with no  -B.  one is 16v rail adapter pre 98 and the other fits 99+ Vitara's with V6  (unsure which is which) below