SANDDOG was right. there is a button to change it from C to F.
but since you said yours got none, i assume that you have the new version AAC pushbutton control unit because the production break point was somewhere in june 1995. the reason why SANDDOG and yours is different.
anyway, you can still change it to degress fahrenheigt, but i always do it by using the handheld tester, and have not done using the pushbutton before...therefore i almost forget. anyway, here it goes:
what you want to do now is to change the variant coding. if you don't get it at first try, just do it again and again, it will not affect or damage your AAC control unit at all. so keep your fingers crossed.
1. ignition "on".
2. set the left display to "high", and the right display to "low".
3. ignition "off".
4. press the "rest" button and simultaneously switch the ignition "on"
> the display on the left that you see (P1) is the first value. the lists of the value that you need is on the bottom, that i make for your input. it is at 1 hertz, and on the right e.g. is "03".
5. press the right temperature selector button. ("blue = < and the red is =>

. got it? the value is changing...right?
left display to "high", and the right display to "low".
6. store the first value by pressing the "rest" button for more than 1 seconds. advance to second value by pressing it again ( i seems to forgot if it was the "rest" button. i need a bit of help from you on this one. please try the other buttons if the "rest" button doesn't work)
7. now the display on the left is "P2" (second value ) at 1 hertz. right e.g. "02".
8. press the right temperature selector button ("blue = < ; red = >

.
9. store the second value by pressing the "rest" button for more than 1 seconds. (same as step 6 including the note...hehehe).
10. ignition "off".
now check if it is okay now...it should be. if not, just do it again, no problem.
here's your 1st numerical value list:
COD - new unprogrammed AAC control module
00 - model 140 (this is your car!)
01 - fahrenheigt display ( just what you are looking for )
03 - celcius display ( don't enter this value )
72 - serial interfaces K1 and K2
2nd numerical value list:
01 - m104 (this is your engine. use this value)
02 - m119 (8 cylinder)
03 - m120 (12 pistons)
04 - om603 (diesel)
+8 - blower bar (you know it)
+128 - smog caption switch
good luck.
tip: if you press the "rest" button for more than 5 seconds, you will see a lot of actual values that may interest you.