$70 sounds extremely steep just for somebody to run a cable through.. However if you want it done perfectly without damage, get a pro to do it, if they damage anything get them to repair/ pay for the damage.
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thats about what I charge for a 2 channel amp EXLCUDING the power kit. $130 with wiring included is the norm
I do this everyday and never had a probelm with panels ( on r31,32,33,34) not going back on right.
if you are in doubt get it done professionally. it will save you the cost of broken panels and trashed harnesses ( if you pinch one when you put the seatbelts back in!) and everyone of us in the business has insurance to cover a f'up IF we make one.
cabling under a car isn't a good idea. RCA cables arent made for outdoors and wont last anymore than 6 months tops. powercable is a similar thing unless its double jacketed.