I don't know if it is on all models, but there is one shure way to find out wheater it is on your car, and that is to locate the antenna-amplifier.
It is situated eighter, in the right-rear upright (between rear-door & -window) or under the cower between headrests and window (in the middle between rear speakers) you might be able to spot it from below.
I have to warn you, the reception is not nearly as good as an ordenary antenna, but of course more practical and not in risk of vandalisme.
Further more the amplifier often breaks down. (bad quality i guess!)
I have had two 155's...the first, a '96 2,0 16V, the amp. was located in the upright, the second (present) a '94 2,5 V6 it is under the cower.