I'm putting a 791xv in soon and wanted to know of any tips/tricks I need to know.

Presently I have an older model Viper alarm (dont' remember model#-need to look it up) in the car that was professionally installed.
I'm aware of the VATS issue (OEM factory alarm) and have the proper resistor value and relay. My current alarm is interfaced with the factory OEM alarm and power door locks for keyless entry.

What I'm wondering is if the older viper alarm harness will plug into the newer Viper brain or I'll have to cut/splice/rewire the whole thing using the newer brain with the currently installed sensors, starter kill and factory alarm interface?
Also any suggestions on where to find the best location to connect the tach wire?

Many thanks to HCCAfan (eBay snyper_audio) for the great deal!

Thanks in advance!