89 b2200 carburated.
the fuel gauge doesn't work and i'm wondering what wires to trace/ where to look... any help?

 

The sending unit is in the tank and i have never seen it be anything else but that. Removing the bed is the prefered method of replacement..

 

Highly likely the issue is the fuel sender in the tank, just old. I actually cleaned my sending unit and BENT the contact arm in a little to restore operation, about two years ago.
Disconnect cable release (if yours has one) and the two rubber hoses at the gas fill assembly (loosen and raise bed slightly [see below] to remove the larger one).
Remove the three 17mm-head bolts on the passenger side bed frame.
Loosen the corresponding bolts on the driver's side, about about 2/3 of the way. Use a 16 inch 2 x 4 on a floor jack to lift up the passenger side of the bed Tighten the jack, and up tilts the bed. I used a 1/4 inch drive ratchet, with 1/4 inch socket and a #2 Phillips bit (and two hands to hold it straight). All mine loosened, but maybe because I'm in Arizona there was no rust; I replaced with 7mm head hex head screws (4mm thread x 8mm long, with a wavy washer). To line up the bed so I could get the three 17mm head bolts back in on the passenger side, the bed needed to be jacked up a little, then I could use my weight to shift the bed a little from one side to the other.

Note: by the way, the new sending unit I special-ordered was bad at the shop (I took in my multimeter to test it) so I never took possession of that. This was a $50 made-in-China part, not from the Mazda dealer. So I fixed mine instead.