Originally Posted by uberwasser

You can browse/search without part numbers.

Once you have the EPC program opening, begin by clicking "Assortment Class" and selecting "Car":



Then click "Model Designation" and look for your model, in this case a 123.193 aka 300TD:



Then under Market, select the appropriate one. If your wagon is a Euro import, select Euro, otherwise North American.



Then select the group, in this case Electrical and Instruments:



Then select the subgroup, in this case Horn:



Now you will get a diagram, select the parts you need and it will show you the part numbers. I selected the screw, and lock washer, that connects the wires to the horn:



Note that you can usually drop the leading letter on the part number. Just use the number.