Splitting an aerial feed

[stig414]

I want to fit an aftermarket head unit to my VW Passat. The HU requires two aerial feeds to work correctly. When fitted to a previous car you fitted a 'stick on aerial to the front screen and plugged in. For the most part it was unoticeable, but still spoilt the clean look I prefer. I could purchase a new stick on unit, but would rather do it using the existing aerial if at all possible.
If I could buy such a unit, would it cause the existing feed to be less effective (it is a good reception at the minute) i.e. a rough 50:50 split with the signal?

Suggestions welcome,
Daz

2003 TDI Passat estate with roof aerial for radio only (no, GSM or GPS)