I believe there is also a circular one under the bonnet
Not sure of it's position on the 1.9 but on our 2.2 it is adjacent to the front n/s suspension turret. This thread gives more detail
The earlier cars don't have the OBDII socket in the drivers footwell, only the round one under the bonnet. Not sure of the changeover date, but probably somewhere around the facelift (04/99).
Cheers R.
Arctic Silver '99 Z3 1.9 & Black '59 Frogeye 1275cc
Mines an Oct/00 facelift and has both connectors... the under dash 'D' shaped socket and the older style under bonnet round connector. However, i've only ever got the round style one under the bonnet to report any diagnostics.