348 Speed Signal

Hi, Im hoping someone can help me!! I have just fitted a Becker Traffic Pro system in my 348 and I need to find a suitable input for the GAL Speed Signal. The extract from the installation guide is:

0.5Hz - 4Khz / square-wave signal
Low - level <0.3v, high - level 5v - 16v

Additionally it says you can connect to the "Mechanical speedometer with intergrated speed sensor in the speedometer cable"

Iam aware that there are speed sensors on each wheel, can these be used and if so where do I locate the correct cable and colour

Any help would be most appreciated as without this input the satellite navigation wont work!!

Thanks,