on my last trailer, i would sometimes have one light that worked and that was the side clearance light on the drivers side. sometimes they would all work. one day i found the problem was in that only light that was burning; the light fixtures had self cutting places to put the wires. the splice that led to the next fixture was there but making a bad contact. i made a wire to wire connection and all worked.
that -4 volts and +8 is trying to tell u something. try pulling that light and see what the voltage is doing. the only way ur going to get numbers like that is if the voltage is not going to ground at that point (i'm assuming the meter leads weren't reversed for this reading; even if they were, ur getting a voltage division).
u mentioned they used those blue slide splices? i've had problems with those as well.