Not sure on the W123, but it should be up under the dash. Look for a writeup here or on benzworld to lead you to it. I think the W123 still uses the silver can style relay. On my W126 it is attached to the same bracket as the cruise control amplifier.

If it works in your parts car, you should be able to swap it over to your driver for a definitive test.

Regarding the dash, I'd personally leave that for a day when you're feeling like He-Man. Removing the dash is not an easy or quick task.

You should be able to get to the blower without removing the dash. Again not sure on the W123, but on my W126 it is accessed under the passenger side dash area. I find it easy to recline the passenger seat all the way and lay on it upside down to get to all that. Makes it easier for my fat lazy ass to get in there than trying to do it squatting down beside the car!

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]