Caravan 1989, 3.0, electrical problem with dash and radio

Hi all,
I have this rediculous electrical problem. it is a Dodge Caravan LE
3.0L Overdrive Trans
my dash meters missbehave sometimes, and my radio is in mono.

the RPM sometimes falls like the engine is off and somes back when i
hit it, or hit a bump. and My coolant temp is flicking side to side,
and my oil pressure sometimes goes to low, the Check gages light comes
on sometimes. I took it apart, even replaced the RPM gage, This tells
me that the RPM is a contact problem somewhere behind the dash, and the
others are a short somewhere in the car.

The radio was missing when i got the car, the wires were there, so i
soldered the connectors from my other caravan 89 and installed the
radio from the other van aswell, it plays in mono and really bad, you
can't distinguish what the words are. Could it be that the speakers ar
8ohm and the radio is for 4ohm, what can i do to fix this?

please anyone had experience with wiring. The car is old so i'm
thinking it could be a worn wire, the inside wiring is in Ok shape
wasn't touched much before.

i like this place, not much activity but no matter, someone is out
there with the answer just waiting for the right question to pop up.
thanks
Ilia