Actually, I borrowed Bill'* heat gun to remove the hot glue from an engine harness. The tails were done outside on a warm summer day. There are three parts to the tails: outer, inner and lens. The outer consists of the black stripes and clear lens, the inner is the structure where the bulb sockets are inserted and the lens is the red, clear and amber sections.
The lens is sandwiched between the outer and inner pieces so you don't really deal with it until the tail light is separated. The inner and outer pieces are held together with butyl rubber and 4 tabs. I began by removing all of the butyl I could with a flat blade screwdriver, awl, and various other instruments... whatever it took to scrape off the access. Next, I very carefully
pried one tab up and left the screwdriver in place to prevent the tab from locking. With another screwdriver, I lightly pried and worked the two pieces starting right next to the tab and working my way outwards from the tab. After working it for a couple of minutes, I could remove the screwdriver from under the tab and work my way to the next one. I took my time and did not force it at all. This is the key, especially the tabs.
If I was to do it again, I would probably use the 90* pick (see below) to further break the seal between the inner and outer pieces. This can help eliminate any damage.
Do you need a heat gun? No, unless you're attempting this when it'* 30*. Can it help? Definately. This will make the butyl rubber more plyable and the two pieces will separate easier. Don't waste time with a hair dryer. You'll just end up with a warm tail light.