Well, I didn't know either, but it appears that a search with a wider scope (i.e. the entire web) turns up this reference in Empeg Newsletter #2:

As you may be aware, the empeg-car has an onboard serial port. As you'd expect, the back of the unit has a PC-style serial connector which you can use to connect the unit to a PC that doesn't have USB support.

As you wouldn't expect, the TX and RX pins of this serial port are also replicated on the car-mounting chassis. This means that, if you have another computer fitted in your car, it can talk to the empeg-car as soon as you've slotted the unit into the dash.

We've been thinking of uses for this. Some are sensible, some are less sensible. Some are downright silly. Some, like these, are stupid, but completely possible. There now follows one of them:

Do you have embarassing corners of your music collection? Are there things that, given the option, you'd not admit to liking because your cool would be so damaged, it'd be a write-off? I do. I'm going to have to get it out in the open and say that I actually like a couple of (deep breath) Hanson and East 17 tunes. What can I say? When noone else is in the car, I have been known to MMM-bop, or even sing tunelessly along to the strains of 'House of love'. Pathetic, isn't it?

For people like me, the empeg-car has something called a Wendy-filter, which is named after a common passenger in my car who doesn't share all my taste in music (not even half, in fact...). When you turn on the Wendy-filter, the empeg-car suddenly forgets all those embarrasing (or partner-annoying) parts of your music collection. In fact, it won't admit they exist at all. They won't appear in any random playlist, and you just can't find them from the user interface. Wendy loves it as she hates Carter USM (pah!). Steve's wife, Diane, loves it because she'd rather not hear Lynyrd Skynyrd.

Ok, so this is easily controlled from the user interface, but you might forget! You may be impressing your new date with your right-on music collection only to be interrupted by Hanson. To prevent such problems, simply wire up a pressure pad underneath the passenger seat (or the seatbelt warning sensor) with a little PIC microprocessor and hey presto! If you have company, you don't hear embarassing tunes. The PIC will send the empeg-car commands to turn the Wendy-filter on when appropriate. What more could you ask for?

This is neat, but I have no PIC-programming skills, so hopefully there's some sort of simple I/O on the Mark2s... (hijacking the cellphone input?)

Alex