Factory stereo question?

[Mark sP]

Here's another Q for you guru's out there. (I have searched but found nothing to answer this Q).

My car has the factory 8 speaker and amp option fitted. The amp is located under the right-hand front seat.

I am upgrading the original Toronto cassette/radio to a Blaunpunkt DJ rad/cass with cd changer.

My question is this: The amp and radio currently has a din to din lead between them. My new head unit does not have a suitable socket to connect this lead to. There are coloured speaker cables behing the dashboard which seem to correspond with speaker connections into the amp. If I connect the speakers into my new head unit, I DO get sound, even though the din lead is not connected to the new radio. Therefore, what does the din lead do? Should I assume that the amp is not actually amplifying the sound?

Any ideas?

TIA
Mark