Well, to be precise: the Technical Manual material under this link does not say late cars should have it in the rear.
Quote: "A radio might have a manual or automatic antenna. Most antennas, especially manual antennas, were positioned on the passenger front fender. When automatic antennas became popular those were frequently installed on the driver side rear fender." And then: "The original Hirschmann automatic antennas that Mercedes sold for front fender mounting on a late 280SL (MB p/n 900 820 04 11) was a Hirschmann 6000-F58:".
I have the 1970 280SL with antenna as factory option and it is in the front.
Without being 100% sure, I think:
- factory - front
- subsequent (dealer on purchase or owner) - front or rear (some might have liked it this way, it is more convenient to install, some antennas, other than original Hirshman, just did not fit in the front? - I do not know)
I would have thought that if you have antenna option on the paint code plate - I would dig more to confirm where it can be/should be installed. If you do not have antenna as factory option, means it is subsequent installation, I would say rear is fine (if you do not have a factory Hirshman, it may not even fit in the front).