I can't recall the procedure for removing the air vent just above the radio.
I've removed the radio and have tried to press the retaining tab "up" but it won't come out.
You almost have it. Take the grill off the top of the dash to access the other clip on that side. Prolly can get by without removing the OBC. More info:
http://www.nmia.com/~dgnrg/page_37.htm
gale
92 735i 5-spd, turbo project finally underway!