First Things First
Spend time with the outside of your vehicle. Probably just as much time, or even more that you did in installing the rig itself. The margin of error in installing antenna mounts is near nil. One simple mistake, and you'll owe the YL big time! Start by removing tail lamp assemblies and looking what's behind them. Pay close attention to fit between the body and the assembly. The gap(s) there may be enough to wedge a good sheet of aluminum. Look for any flat surfaces that you can drill and bolt through. Keep in mind that for the heavy duty mounts, you'll have to brace BEHIND the body as well. Don't overlook decorative plates, trim, anything that is screwed into the body. It's always easier if the holes are already present, but don't be afraid of drilling -- you'll be surprised at just how easy it is to drill in the family sedan.
You've found a place to install your homebrew mount, it has a flat place to mount it, but there are some curves and custom shapes that will need to be cut. Don't worry! ( Are you tired of me saying that yet?? ) All it takes is some paper and pencil. What??
Yes, that's right! Don't even think about carving into your ride just yet, you still have some planning and development to do. Get yourself some poster board and some pencils. You'll be making a paper mount and backing first and use that as a template for cutting and drilling your sheet metal. After some playing around, my paper templates looked like this:
Notice that I had some tough curves inside the tail light assembly. The template on the left is the backing that braces behind the body for the mount. The backing template is rightside up BTW. The one on the right is the actual mount that fits inbetween the tail light and the body. It's backwards and upside down tho'. The right side of the mount is what goes inside the body, and the left will be outside. Keep trimming and it will be evident on where to mark the holes to be drilled for the bolts.
Cutting out the Mount
OK, now it's time to lay the templates out and outline them onto your sheet metal. What kind of sheet metal?? Well I went down to my local industrial metal supply house and spent $2 USD for a piece of .22" T-6061 aluminum sheet. That's about as thick as I can go with using standard hand tools. YMMV, so use your best judgement. Here's some cutting pix:
Doing the actual installation is the fun part! If you were careful in designing your templates, then there shouldn't be any surprises when trying to install it into it's final resting place. When choosing bolts, nuts, washers, etc. I went for broke and used only stainless steel hardware. The mount went through a rather wet Southern California winter just fine. Also, when tightening it for the first time GO SLOW! Don't crank it all the way tight too fast, you could end up creating creases in the body. From time to time, check to make sure that it isn't working it's way loose. Once it was installed, I could actually stand on it and it would support 195 pounds! Not too bad for $2! With some additional aluminum angle brackets, I was able to have it fully articulated. That way I could be sure the screwdriver antenna was plumb. Also, since all the holes in the body were hidden, when the mount was removed, there was no indication that anything had been installed.
Here's some more pictures: