You need an adaptor plate/facia plate and wiring. Then pretty much everything fits, single or double din! It's up to you! (even one of those Android based NAV units might be worth a look).
It's early and I've done a quick a dirty search for parts so wont guarantee they are the right ones, but something to start off your search in any case:
The unit in newer cars is a double din hole, you just need the right trim pieces to fill around it like this:
Pioneer Double-DIN Installation Kit for VW T5 Multivan: Amazon.co.uk: Electronics
Mind you in the case of the multi it looks like it actually adds in the internal frame too... in either case there is a kit for that.
This leaves you with the option for any double or single din radio ever made.
A quick and dirty search showed this:
Amazon.com : Stereo Wire Harness VW Jetta Double Din 02 2002 Car Radio Wiring Installation Parts : Vehicle Wiring Harnesses : Car Electronics
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