I have been a long time fan of Pioneer car audio and they do a range of 'drop-in' replacement speakers that fit several ranges of cars' standard speakers. I did this on my 300E's rear speakers. Not sure what the front ones are but they sound better the standards so guess they have been upgraded by a previous owner.

I am fairly sure they were these ones:
http://www.pioneer.eu/eur/products/25/132/201/TS-H107/index.html
Look under 'car application' to see what I mean about generic car fitting.

They cost me about £50 UK and as per the image come shipped with fitting plates (in case they're needed). If these are not suitable for your wagon I'd be certain Pioneer have another type that would fit:

http://www.pioneer.eu/eur/products/25/132/201/overview.html?__QType_UNI_CF_SW=1

I recently helped a mate replace all his standard speakers on his Vauxhall Astra using speakers from this range.

And no, I don't have shares in Pioneer!

Regards,

Glyn B.