Found my air pump relay in the RCM had fried to the point where it wouldn't make contact after filing it clean (unless I bent the copper arm and made manual contact with that!).

If you wish to give your own RCM relay replacement a try (I'm forced by IL refusing to renew my plates without an emissions pass), this eBay seller has individual relays for sale at $3.99 + $2 shipping:
American Zettler AZ970E-1A-12D Mercedes Benz or Chrysler Crossfire relay | eBay
for our beloved American Zettler AZ970E-1A-12D.
All the other parts houses had big minimum orders - hope this helps someone.