[Lyle O]

If you checked the relay already, it pretty much must be the temp switch. I would have to check DME circuit diagram, as that could be the only other possibility I think.

To change the switch, you don't need to drain the oil, but you will have some spillage and will need to keep the lines "up" after disconnecting. Also, remember not to tip the cooler too much.

You can avoid the whole disconnection if you think it easier to remove the front bumper. Then you can access it from the front while the cooler is "in situ". Just another option.

Running with the fan on all the time won't hurt anything except cause more wear on a 34 year old fan motor. Mine wore out years ago, so you may already be on borrowed time on that front. If you are in Canada or somewhere north where the heat is not as big an issue, you might consider disconnecting it. Otherwise, you can let it go until convenient to replace. Also, consider going to a fan switch which comes on at a lower temp. I did this and feel better about the fan coming on earlier when needed. There are several threads about it if you do a search.
Good luck.