i use DEI relays makers of viper alarms 30/40 amp for each compressor never had a problem and i put them tru water when i wash the truck they only cost 5 bucks at my local sterio shop or try a bosch and add black tape to it lol so water or moisture would get in
ford starter solenoids are ment to run a few seconds not a long time. my buddy works on RVs, and he uses a RV solenoid that they use to charge batteries. it looks like a ford solenoid, but it lasts alot longer.
go to grainger get a heavy duty, they use em for golf carts and rec vehicles, come with a one year warrenty. they cost a bit but its worth it. i ve had mine for 9 years maintnance free.
I use two diferent kinds one is a pac-80 bat isolater good power handling good weather resistance at $30 or for higher amp aplication i use a batt isolater solinoid from my local auto electric suply company better power handling better weather resistance at $50. the key is they are designed for continuous use where standard starter solinoids are designed for short Intervals and the bosh relays are not watter resistant and dont carry high amps.
i got a sealed continuous duty solenoid from my local truck parts place and its butter. Its self grounding so there are fewer wires involved which i like alot and after i mount it in its final location im gonna spray it with some krylon and call it a day. thanks for all the help guys.