2008 with a Shaker 500
Thanks all!
Ok, cool. Now to narrow it down a bit -- What's your budget? Do you care about having a CD player? Or volume buttons or a volume knob? And, of course, have you looked at Crutchfield? Crutchfield has probably one of the best websites for audio gear out there. Between the interface, categories, and filters, you can pretty much narrow it down to exactly what you want.
I would look for something with 6 channels (2 front, 2 rear, 2 subs) to allow you to use the door subs without too much hassle. The stereo will have 6 female RCA plugs either on the back or as part of a dongle that plugs into the back of the unit.
You'll also want a 5V regulator (or 12V to 5V converter), like this one:
https://www.crutchfield.com/p_120AFDI5V/Axxess-AFDI-5V.html
Aftermarket stereos use a 12V signal to turn on remote accessories, like external ampliriers, but the factory amplifiers for the door subs (in the driver's side footwell) are designed for a 5V signal from the Shaker head unit. Don't know if it damages anything, but if you give them 12V, you'll get a loud 'POP' when you turn on the radio. Get the converter, and it'll step down the voltage from the aftermarket stereo to 5V.
Crutchfield will automatically pick the installation gear you need and for the '08 w/ the Shaker you have two options, the PAC C2R FRD1 interface, or the iDatalink Maestro RR. PAC is the cheaper of the two, but doesn't have as many features listed.
https://www.crutchfield.com/p_127C2RFRD1/PAC-C2R-FRD1-Wiring-Interface.html
https://www.crutchfield.com/p_794ADSMRR/iDatalink-Maestro-RR-Interface-Module.html
Crutchfield says Bluetooth isn't supported by the PAC audio interface, but I think they just mean if you had a factory bluetooth module (which the Shaker 500 didn't have in '08 anyway). But the aftermarket radio will have its own bluetooth and microphone so I wouldn't worry about that.
One thing the iDatalink is supposed to give you is access to vehicle data (performance gauges, tire pressures) on your head unit (if the HU has an iDatalink serial port), but I've read that it's pretty spotty on the '07-09 cars (05-06 aren't supported at all), so IMHO you're better off saving your money and go for the PAC interface.