So besides the HIDs, I would not have minded having the Ambient lighting from the ST3. Had it in my Fiesta, liked it, but.... Don't think it was worth the price of admission, especially since I can get a similar effect for $35 :)
So I hit up e-bay and picked up a roll of waterproof LED strip with controller and remote for around $30.
Then I went to Radio Shack and picked up a 12VDC kind of like this one but make sure that the fuse is rated for at least 10A.
Enercell 12V/10A Solderless Vehicle Adapter Plug : Adapter Plugs | RadioShack.com
You could certainly do this more elegantly but the lighter power kicks on when I unlock the doors and goes off about 10 minutes after leaving the vehicle, this works well enough for me.
Next step, route the controller in either passenger or driver footwell so you can have just the IR reciever sticking out from under the plastic (note the little white wire coming out under the center console plastic.
I started in the pass footwell, went around the side and under the scuffplate, then under the pass then driver seats, then under the driver scuff plate and then finally terminating in the driver footwell. Making a big G.
Stock Color (but waaaaaay brighter)
Then a couple of shots from the back.
The controller has a few modes, like fade, strobe etc... then about 12 colors to select from. It is no where near as adjustable as the ambient lighting, but if you spend a little more, the controllers get much better.
Still, for the effect and price, can't beat it for the sheer ease and cost.
As an alternative, instead of the LED strip, you could get nearly as much brightness and a whole lot less work with 4 RGB Surface mount LEDs from Oznium, and a controller, but you will pay more.