While changing head unit, I accidentally snapped the little plastic pin that is in the middle of the plug,meaning it no longer plugs into the back of the head unit. How can I fix this?

Thanks

chop the head/unit off IF you can source another. They are like the old ariel leads for a tv. If not then I guess your looking at a new ariel lead.

Any idea if there are any of eBay? Or how to fit it?

http://www.ebay.com/itm/Euro-Car-Plug-AM-FM-Radio-Antenna-Cable-Adapter-Charger-/151472544259?pt=LH_DefaultDomain_0&hash=item2344779a03

you mean that part ?
(this is not the exact part to fit your radio, just an example to understand)

Yeah that's it exactly , mines black plastic

less than £3.00 on ebay.pop to fit.

Would I need to cut off the current one or does it pop on? If it just pops on might I have the same problem as I do now, that it just pulls straight off?

have you ever fitted a tv ariel plug?...same kinda thing.