First of all the ITPS acts as a voltage regulator
Second, I think the best way to power your LCD is through the PSU. it the the best regulated, controlled and protected, and it will only consume around 7 Watts. Why do u want it connected to the headunit?? there must be some good reason for that. Anyways, heres a discussion of how to power it throught the PSU:
Xenarc 700TS / hardwire power.