Electrical Nightmare

I am having some problems with the electrical system of my 1998 Windstar 3.8 Liter.

I have replaced three turn signal blinker relays under the dash and two heater fan motor relays in the last year on this van. The last turn signal relay lasted about three weeks before it went bad and quit working.

Right now I do not have turn signals or control of the heater fan at all. Before I go spend hundreds of dollars to have this diagnosed, does anybody have any ideas of what could be killing these relays? There must be a short or ground, but any help would be appreciated.

Do you have a trailer hooked up or maybe the trailer wiring harness/connector is wet or corroded? Genuine Ford relays?
You have probably checked the connector tabs that the relays plug into? I have seen them get corroded up with a chalky deposit. You should run a very thin file into them, like the old school point files. (even a small knife)
After that the next thing to check may be the main ground for all the relays in the fuse box. For my 97 (yours should be similar) it's called G201 and it's located (according to Alldata) behind the RH side of the instrument panel. There also is a huge 15-20 wire ground splice (S205) in that area as well.