Well, at least they're consistent - they treat everyone like crap.
I wonder what the difference is with the 3-wire version. Heaters split into 3 zones rather than 2, maybe?
I just looked in the RN catalog, and I only see one heated windscreen: RNC927 for $659. Where did you find the one for $500? Unfortunately that may be the best deal. Might also want to check with George at RDS.