WRYR258

Hello all, First post here. A few months ago I picked up a MXT500 for a NV desert trip. I was originally looking at the Kenwood TK-7360HK for the off-road race channels (151.625 - 154.490) but after adding a handheld to the cart I realized I was spending a lot and would have very few opportunities to talk on these channels. I have three sets of handhelds (Midland, Motorola and Cobra) that I have collected over the years and realized they are all GMRS so the MXT500 was a natural and lower cost option. So far its worked great! My install was rather hasty in an effort to get there. I pulled power from the 12v plug (cigarette lighter) into a terminal block, then ran the coax through the back slider. I placed some scrap Pelican case foam in the window frame and closed the slide to seal it up. I used the magnetic mount and placed it to the rear of the roof by the third brake light. I ran the short 2.1 dB antenna on the road and switched to a 6 dB whip in the desert. Q: Antenna location. Would I get better SNR (sorry I am old school, I know that's not the proper term) if the antenna was mounted more in the center of the roof? Does the magnetic mount have to be on the actual roof or could it be a few inches above, on something like a light bar/roof rack? Q: Coax run. I don't have any larger gaps or wire harness seals I can easily run the cable through. I don't want to hack the factory cable. I don't have a crimp tool (BNC?). Is there a firewall/bulkhead pass-through fitting for this cable? Does adding length with an extension cable affect performance if kept to a minimum length? TIA