I had this exact problem although my fan would always work (only on high). I purchased an automatic ECC from the volvo dealer in Olympia washington. They had the best price. I've called every salvage yard this side of the missippi and had no luck. Many manual ECC's but no automatic which is what the '93 850 has. I'm glad I got it new because according to dealer there was 3 upgrades to the unit since production. I belive I paid $600 and I installed it myself. You just need to pull out the glove compartment to access the wire harnesses on the back of the ECC.

I hope this helps.

p.s. maybe check your blower fuse (i think it's #4 or #5).

-jack