I just bought a used Alpine 7893 with the Vdrive internal amplifier, but it didn't come with an instruction manual. Some sites say you need to run a wire directly from the battery to the power wire on the HU and others say you can just use the wire from the factory harness (like any other HU). I would rather use the factory harness to power the HU, but not if it won't power the internal amp to its full potential. So can any one help me out! Thanks :D

bump!

Connect it DIRECTLY to the battery...the constant power lead that is. The switched power lead can go through the wiring harness as usual.

If you are using the HU power (internal amp) you definately want to run power directly to the battery. It isn't really that big of a deal to do.

offset

I have a 7894 V-Drive and I connected it directly to the battery as Alpine requires. You should do the same, don't cut corners on the install if you want it to work right.

I just ran all the wires for my speakers and amp before so I am just tryin to be lazy... Thanks for the input guys! Guess I will have to run a 16 gauge wire to finish the install off. Mulder how do you like the 7894?

Use 12 ga. not 16. Be sure to fuse the wire (20A) as close to the battery as possible, this is for safety in case that wire shorts between the HU and battery.
I've been happy with the 7894 since I installed it about 4 years ago. IIRC the main (only) difference from the 7893 is MP3 capability. I did use it for MP3's at one time, but since I added the XM tuner a couple of years ago I don't listen to much besides that.
My only real complaint is the display, in bright daylight it washes out and is unreadable.