It is possible to do this without taking out the antenna but it is a little tight. Anyway all you need to do is (if it is an orginal Antenna) open the case of the antenna. Once open, you will notice that there is a small copperbased circuit board on top of the machanics. That printed circuit board is usually the probem. Most of the time you can go and purchase one from a good local Benz repair place for about $10-$20 bucks, you might be able to find one on line. Anyway once you remove it.. you will have, if I remember three leads which connect to that board. It is a 12 volt system, one is ground and the other two is for the up and down motion if my memory serves me correct. I replaced mine about three months ago and it works like a champ. did not even need to adjust it. I have a 1988 300 SE with 230,000 miles on it..... If you still feel unconfortable you can purchase a file from this guy here.
http://www.repairflix.com/cart.php?cat=Mercedes+123+Chassis+Procedures he has several videos that may help you. I purchased several and thay all helped me with exception of the instrument removal one. There is a better way... In the picture, right where he is adjusting the antenna is where the circuit board is. Good luck ....