I just found this link... The company PAC has been mentioned in the forums before, but they have some new products that are even more interesting for empeg applications.
This is an interface to a wired remote, which will send out learned-IR.
If you go to their main page (eliminate everything after ".net") you'll be able to click on info for their new SWI-X product - a universal steering-wheel remote adapter. This may be able to work for people who haven't found products to match their specific vehicles yet.
And lastly, these other guys,
Peripheral also make adapters for many vehicles (both steering wheel remote as well as changer adapters and LINE-IN converters for decks that don't come so equipped)
Bruno