Couple things to know when installing a stereo:
It needs a solid ground connection bare piece of metal is the best, try not to use the factory ground in the radio harness. Black on the alpine.
It needs a constant 12 volt supply, yellow wire on the alpine.
It needs a switched 12volt supply, red on the alpine.
Then there will be a blue/white or light blue, it should say remote out, this will connect to the remote turn on of any amp, or signal processor.
The rest of the wires will be other accessories and speaker wires. The speaker wires you will need a + and – to make music. Since the alpine is a 4 channel out, 8 sets of wires, ones with a black strip are the negatives.
Since your tt is factory amplified, is must utilize a remote lead (remote turn on), to be activated by the source unit (head unit). Here is what you should do:
Get a multimeter, preferably a digital one
Buy a mounting kit, and factory interface adapter plug
Remove the dash trim,
Make sure you purchase a factory interface plug connector. This is a plastic plug that connects using the factory harness, where by eliminating wire cutting.
Double check all the factory wires, for illumination, 12v, switched 12v, power antenna, ground, remote turn on.
The adapter plug is very simple to wire, match all the corresponding colours all but ground, you provide the ground as described earlier
Tape the wire connections very well, it is recommended that you solder them, but only real hardcore guys do this. On the factory out you will find the power antenna lead, remote turn on, speaker wires, power, switched, and ground, as well as illumination.
The only two wires you will not use are ground and illumination.
If you need more help, just post,