Like allready said you dotn have to.
I tried with a hair dryer as i didnt want to use the oven either, but it was taking a VERY long time, and when pulling them apart the mastic was still rather strong and stringy.
Put them in the oven on about 75 degrees, shut the door and leave for 5 mins, check them and try get it apart, if its still strong do another 5 mins (you will probs smell when its ready lmao)
ALOT easier int he oven jst make sure you keep checking, i meen when i done it one of the plastic brackets bent abit under the heat but when i was heating them back up to put the glass back on i just made sure i bent it back into place when it was hot.
Also if you can, maybe put a heat proof towel under and over the top of the headligth, jst to stop the oven trys maybe melting into the plastic.
Dont do what i done tho and use a tea towel lol flaming tea towels are a right nightmare!
Its easy enough tho mate, but i cant stress enough KEEP CHECKING! even stay in the kitchen jst to be sure