Fan wiring help: Volkswagen switch + AC/on.

Are you using something like Luke's article as a reference?

If the thermal switch can switch 12 volts (I think his schematic showed it doing just that) then it's easy. You can tap into the wire to the positive side of the AC clutch, and run it to the same terminal on the relay that your VW switch goes to(86 in most folk's diagrams, if they are sticking with conventional nomenclature).
I might add a blocking diode to be real anal if someone envisions feedback to the switch being an issue. I think that might be overkill.

If you're switching ground with the VW switch: you'd have to plumb a second relay for the AC. On the second relay: I'd just have the AC wire at 86, ground at 85, ground at 30 and 87 would be a ground output. I'd run that 87 wire to the same wire on the first relay that your ground from the VW switch goes to (you're only using the second relay to convert the 12 volt A/C-pulse to a ground-pulse that's compatible with your VW switch's polarity).

I'm sorry - that was clear as mud, huh?

That sounds like a plan (we were thinking of the same fan write up - that article's on Luke's site).

Since he indicates that the VW switch is isolated and can pass 12 volts, your idea should work like a charm.

Good luck!