First, I just want to point out that if you use a real Lambo RNS-E then make sure you have a Lambo nav DVD. If you use a Audi RNS-E with a trimmed down faceplate, then use an Audi DVD.
The GPS antenna can be mounted under the passenger's side dash. There is a small metal dash support under there that is the perfect width to mount the GPS antenna on top of. It will have a clear view to the sky with only the leather dash and the windshield above it.
The wiring part will be really easy. The Lambo does not use an external amp at all, and has no rear speakers. So, use a non-bose type wiring harness. You can also remove all the rear-amp wires from the harness if you wish as well as all the CD changer wires. Actually, there's not that many wires you need in a 2004 Gallardo:
-Front speakers
-CAN H and L
-Power
-Ground
-Alarm Ground
-K-line
These are all located in the spots you would expect for a sym2 radio.
Of course, you can connect a bluetooth module to this car as well by making all the typical connections to the RNS-E. The dome light is the same one used in the A4 cab so it has a spot for the microphone.
A word of warning though: You will not be able to connect VAG to this car and recode it. You need a custom "adapter" cable so that the VAG cable can connect properly to the K-line and CANbus in the OBDII plug.
When you code the RNS-E obviously use the Lambo code, and the sound type should be "linear". (0100xxx) The cluster coding would be as expected (channel 62 = 5).