It is unlikely that you can install a bulb "inside" your taillights. The toad tailights are LED fixtures and there is no inside. On my toad the dealer strongly advised against any set up that introduced coach power to my toad taillights, even with diodes. With LED there is no bulb and if the wiring causes a problem in the toad taillight the whole unit requires replacement at over $500.00 per side.
My solution was to use wireless tow lights. They operate independent of the car system. My wireless lights are LED and super bright. I place inside the rear window on the upholstered deck so no danger of them getting damaged or falling off. Each side takes 4 AA rechargeables and one charge will last for a couple of days driving time. I have an extra set of batteries and put fully charged batteries in every morning when traveling.
Quick, easy, legal, inexpensive and no one had to hack into the wiring harness of my new car. I highly recommend
Here is a link, unfortunately I don't know how to make a clicky so cut and paste to your address bar.
http://www.blazer-international.com/video.php?video=2