Fuel Guage help

[65-72 chevytoys]

Last weekend I found the break in the wire that caused the fuel guage to go past E i.e. no resistance. Spliced the wire back together and connected the tan wire into the fuse block. Now the guage stays on empty after putting about 5 gal in the tank. I started tracing that same tan wire from where I found the break to the back along the frame rail. I could not find anywhere it was grounding out, but I could only go to where the wire went over the top of the tank. I could not reach in between the blazer tank and bottom of the bed to where it connects to the sending unit. Anyone know how to check the wiring or the fuel sending unit without dropping the tank? I want to check of the wire is grounding out or if the sending unit is bad, but I can't get to it. Any ideas?