What does the middle position on the ant. switch do?
 

Ok, the antenna switch has a "down all the way" position, a "full height" position, and a middle position.

What does it do?

Only puts the antenna part way up. Really. This is usefull if you are in an area with strong signals that interfere with each other.

It also lets you "lock" the antenna in any height position you desire. Use the "up" switch to raise the antenna and the "down" switch to lower. When the height is at the desired position, depress the middle switch and the antenna will stay at that height. So, you can set the antenna to just be two or three inches raised if you want. Kinda nice. Only problem is that when the radio is switched off, or you remove the key, the antenna goes all the way down and will not "remember" the height you selected.

The handy little card (OE) of radio information for my '85 300TD says:
 

that when the switch is set in the middle position, the antenna is supposed to automatically rise to the ideal length for best reception of whatever station the radio is tuned to, sort of like the local vs. distant switches on a lot of radios, but on my car, on the middle position the antenna never seems to go up far enough for decent reception of anything except stations within a mile or so, so I always have it set on the "all the way up" setting. But then, after all, it's going to turn 333,333 miles sometime next month.

Play with it. Around here there are a lot of mountains and water which creates multipathing. I can get better signal sometimes by moving it all the way up (can actually increase the volume sometimes) and other times can reduce static by moving it down a bit (less signal, and this filters out the weaker multipathed signal)

Re: What does the middle position on the ant. switch do?
 

It's something my Dad would like.. Another button to F with that doesn't do much..:D