91 ford econoline van

ok heres a tricky one.  i got a 91 econoline van in the shop today for a new headunit.  well after i wired it all up, i got no output from speakers.  so i start testing pairs of speaker wires for ohm loads and i got weird numbers like 55 and 67 ohms.  so i looked behind the ford harness(factory side).  this harness has 8 wires, two for each speaker, but on the factory side it only had five.  four for the positive wires and ground so i realize the speakers are common ground.  so i grounded all 4 negative speaker wires and still got nothing! i looked around and found what im pretty sure is a factory amp way back in the dash to the right.  im wondering if there is a way to bypass that amp or if I have no choice but to run new speaker wires to each speaker.  i set her up an appiontment for sunday.  i couldnt do it today bc i had other appointments waiting for me.  what was supposed to be a 20 min headunit install is now possibly an hour of running speaker wires.  if anyone knows a way to bypass that amp please let me know!  thanks

Jimmy