I don't have a 123 body any more, but I don't think it's a big deal. The mechanic who found the problem for me spent two hours finding and fixing it.
Be sure to disconnect the battery, then I think it's a matter of a couple of screws and some careful tugging.
This was a problem on my 280E, and I believe that it was caused by the drain hole under the hood hinge being clogged. All of the water coming off the windshield has to drain there.