After discussion with the owner it was decided to install the main antenna supporting the voice radio into the middle of the roof. This will allow the optimal radiation pattern for the antenna. However, this would require removing the head liner from the vehicle to allow access to the center of the roof. A compromise was found by installing the antenna directly above the rear passenger dome light which was only slightly off-center to the vehicle.
The Dome light is easily removed by gently pulling on it. It will fall out and be held by it's supply cable. Remove the supply cable by pressing the clips found on either side of the connector. Remove the light and put it in a safe place for later restoration.
Now you are left with a rectangular space. Find the center of the space by drawing an X from the diagonally opposite corners of the hole. Drill a small pilot hole at the center of the X from the inside to the outside of the car.
Now drill out the hole you've made with increasingly larger drill bits until you have a hole that will correctly accept your antenna mount. My NMO mount required a 3/8ths hole but others require 1/2 inch or even 3/4 inch holes. Increasing the size of the hole in steps makes for a neater hole and reduces the risk of buckling the metal with the torque of the drill.
Clean up any shards and splinters with a slightly larger drill bit but be careful not to create a larger hole. Install the antenna mount into the hole you've made making sure to weather seal it where necessary.
Now we need to pull the coax from the mount into the back of the vehicle (the radio will be going into a pocket in the trunk). Feed a snake (I used a wire coat hanger) from the back of the car to the antenna mount, attach the coax to it and then gently pull it towards the back of the car.
Depending upon the style of the mount used you may have to insulate the bottom parts of the mount so as to prevent them shorting on the light. A strip of duct tape would be ideal for this. As you can see, my mount had a plastic cover already. Reconnect the dome light and reinstall it back into its hole. We are finished with this part of the installation.
I did say that I installed 3 antennas. For the remaining 2 I used a combined GPS antenna with NMO mount. This was installed at the back drivers side of the roof. I didn't get any pictures of the install as it started to rain just as I had drilled my pilot hole. But, here's the finished product ...
The flat part at the rear of the mount is the GPS antenna whilst the front part is an NMO mount. As you can see, the rain made taking pictures of the install impossible.