The problem with routing power cables next to wiring harnesses is that the wiring harness becomes a simple dipole antenna and any RF noise you have in the body wiring harness / chassis computers of the vehicle - comes through the radio.
Power wires are inductors - just think about the experiment you did while in elementary school where you wrapped a piece of insulated copper wire around a 10 penny nail. With no power applied, nothing happens when you touch other nails.
But when you apply power it energizes the nail making it into a simple motor structure - magnet.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmut.html
http://electronics.howstuffworks.com/motor1.htm
The key is knowing where to drill the hole and using the proper grommet.
If you are using a 50 watt - 2 meter mobile, the power and ground wires will be either 10 or 12 gauge - with fuses on both wires. Some vehicles - I think the Dodge Caravan is one - you can't drill a hole through the firewall. I can't remember the reason why - but I know that it is not advisable.
Nor is it advisable to route the wires through the door and fender.
As I tell all my new hams - you are a ham now, it is up to you to do the right thing and to do it properly. Unfortunately I can't proclaim that I practice what I preach.
All the vehicles I operate are leased vehicles or loaners and I am not allowed to do anything to them except drive them and maintain them. That is the price of FREE...
The owner got upset when I put a mag mount antenna on the roof of one.
But he has every right to be upset - because I bought a used Larsen mag mount from a idiot ham guy and the magnet was no good and I got caught in a small cross wind one day and the antenna blew off the roof 3 times in 3 miles and whacked the door in a couple of spots and left big dents.
When I asked for my money back - the ham guy got upset and I ended up loosing $80 when I thought he gave me a $100 instead of a $20 and I gave him my $100 bill and he gave me $20 back.. The moral of the story is - some guys don't get rid of anything until it is junk and even then they try to rip you off any way they can@!