Line-level converter - Impedance??

[helidan]

I understand many people are now 'piggy-backing' line out converters on to the car's oem stereo front speaker wires or using amplifiers with high level input but I was wondering how this method of connection affects the impedance the head unit sees? Normally it would see a nominal 4-ohms but do LOC's/high level inputs on amps change this by much?

Thanks for any info.