Haunted electrical-doors locks engage & speedometer fluctuates

Its a common problem in this forum but nobody found a solution yet. The symptoms on my 160,000 mile 2008 LTZ include: engaging door locks, AC blows warm air, speedometer fluctuates or goes to zero. Engine was replaced 6 months ago and thought dealer would have noticed frayed wiring then. Dealer says the problem is the wiring harness connecting the alternator and battery and engine costing $850. Does this seem pricey?

Actually it does not look bad, if it includes labor

I had an 07 Tahoe previous to this and had crazy issues where the truck lost all power as if the battery was disconnected after shut down. On the Tahoe forums others with this issue also had the same problems you mentioned.

There is a mega-fuse on the passenger-side underhood mounted on the firewall. 12v comes off the battery and goes into this. A lot of people mentioned theirs was looking burnt and replacing it fixed all their electrical gremlins. Didn't fix my problem tho...I traded it in and forgot to mention it to the stealership 

Dunno - might be worth 10 minutes to check.