I used a 5' or 6' category 5e patch cord and a RJ45 coupler. Zero issues with my TM-V71A. Twisted pair cabling is balanced so I would think it should be helpful in rejecting external noise.I strapped mine under my seat springs as well. Normal RJ45 extension for the mic is said to pick up and put a bit of interference static on your TX. I have not heard it, and it should vary depending on environment, but I read about it while researching a more remote mount. Fancy Mic extension cords are available but pricey so the home brew route is probably worth trying.
I think you'd only be able to keep noise out with well shielded cable or fancy inverted signal merging. The official extension cable was like $70 bucks when I looked and some guy was making custom length cables for about the same but had stopped. Thats a lot of money to me and makes $10 bucks worth of standard network stuff worth a shot I have my mic plugged in under the seat with the stock cable. Its usable, but tight and I'd rather it have a couple more feet of cord. If my friends continue to convert to HAM I'll have to do something myself.
Here is my Yaesu 8800 in an 80 series. Main body is in the left side of the driver's footwell. If you did the grey wire mod, the radio is right in front of that wire harness.