Get a can of DeOxit
for the electrical connections. It'll do a great job of removing oxidation, dirt, and grease, and help keep those electrical gremlins away. The PO on my boat left the cover on one year with water under it, causing both mildew problems and electrical corrosion. To get all the gauges working and reading correctly again, I disconnected all the connectors, and hit'em with a shot of DeOxit which solved most of the problems. Afterwards, I had to pull the gauge cluster, loosen each of the wire nuts, hit'em with a bit more, and tightened everything back up. My voltmeter now reads the correct voltage, the temp meter the right temp, and the tach stopped going crazy every now and then. Hitting the bilge pump no longer causes all the gauges to drop by 20%.
It's a $20 can, but it's sure worth the effort.