1997 Jeep Grand Cherokee Laredo - Electrical Problems

1997 Jeep Grand Cherokee Laredo - 201,000 KMS. Up until this point, this Jeep was running perfectly. Everything was working fine. About a week ago, after the Jeep had been running for about a minute, the temp gauge shot all the way over and the "CHECK GAUGES" light started to flash. It had not been running long enough to overheat, so I immediately assumed that the coolant temp sensor had failed. As I continued to drive however, I noticed the instrument, radio, and overhead console lamps flashing erratically. The dimmer slide beside the headlamp switch was doing some strange things. It still turned my interior lamps on at the correct setting, but as I slid the dimmer down, the interior lamps shut off (as they should) and the instument, radio and overhead console lamps flash bright and stay there instead of dimming. I turned on my wipers, and no matter where I set the delay, they ran at low speed. (High speed works if I turn it to that setting, but they run at low speed throughout the intermittent range.) We hooked it up to a friends scanner, and there were no codes logged. It read the engine temp as 101 degrees (which is where this Jeep always runs), even though the gauge in the dash was still pinned at over 125 degrees. I took it to a dealer, and after an hour and a half, all he could tell me was that the signal from the coolant temp sensor to the PCM was good, and the signal from the PCM the cluster was also good. He couldn't explain about the wipers or the instrument lamps. I've disconnected the headlamp switch, and the wipers still have no delay and the gauge still reads off the charts. I've disconnected the instrument cluster, and the dimmer and wipers still act up. I can't help but think this is all related as it all happened at once, but it's driving me nuts. I think it may be a faulty ground somewhere, but where do I start? PLEASE HELP!