electrical short after fuel pump install

Hey guys I have a strange electrical problem that I need
help with. Last night I finished installing a 255 in tank
pump and I went to prime the system and the lights started
dimming, there is a short somewhere. I unplugged the
sending unit and the short goes away and the pump primes.
Can a sending unit itself cause a short or is it the wiring
coming into the sending unit, where is the wire coming from
and how can I isolate this circuit, is it coming straight from
the dash behind the fuel guage, thanks in advance.