You have an EXTERNAL antenna?! I thought all E36s had the imprinted rear window type antennas? Anyways, regardless of antenna location - make sure you get the "good tint" and not that purple crap that bubbles up in 6 months. That $50 off coupon is good - but don't cheap out for the worst tint...you'll hate it.
I tinted my old Civic at Tint King and paid around $200 for everything (tax too) - got the "higher grade" tint and no purpling and no bubbling appeared for 8 years.
I went to a place in Scarborough to tint my current ride. Not a bad job - but they could have been a bit neater on the rear window. I think it cost me $140.
Just remember NEVER use ammonia based window cleaners such as WINDEX to clean tinted windows. Just water is fine.