http://www.ebay.co.uk/itm/Witter-Fi...-Fits-Fiat-Croma-Estate-2005-On-/231035852157

not sure if wireing is plug n play and can't ask seller a question

The wiring is not "plug n play" You would also have to cut the underside of the bumper.

are there and mounting holes predrilled on croma? or do you need to drill new holes?

I didn`t actually see the towbar put on my car, but as far as I am aware there are holes already in the chassis.
It was possible to order the car new with a towbar as an optional extra so I can only see the holes being pre- drilled to allow quick fitting, assuming the towbar was fitted on the production line.

Depends on the towbar make.

With mine (a Brink) I had to drill the holes but all holes were simple to drill, no nasty right angled access/space issues.