Don't go for 13cm replacements but the 16.5cm diameters instead. Click on Auto Audio Acessories link at the top of this page. They can supply you 16.5cm speaker adaptors. Alternatively there is another place in Ireland who supplied me as AAA didn't have any at the time. If you do a search through previous speaker threads you will find the name. You can then buy a speaker component set from Halfords or where ever for around £70 - Alpine, Infinity or JBL. You will obviously have to remove door cards, shave a tiny amount of metal from the speaker aperture in the door at the top. You can still retain the original mounting holes if you so wish to reinstall the original crappy speakers at a later date. You may need to shave a few slithers off the inside face of the speaker grill on the door card with a stanley knife, be careful! You will also need to replace the tweeter on the dash with the good new tweeter and locate the crossover boxes somewhere under the dash say around the glovebox. The rest is easy. The improvement in sound is huge even if you don't buy audiophile speakers costing the earth but some fairly average £75+ component set speakers from the likes of Alpine, Infinity or JBL.
I was going to go for 13cms but then for pretty much the same amount of work you can fit 16.5cm speakers. The sound is so much better. I have 13cm 2 way JBL in the back which although good are not as capable as the Infinity 16.5cm component set in the front.
If you feel it is bit beyond you as finding the wires can be tricky then Halfords will probably fit them for you for about £35 provided you bought them there. They were doing the Infinty 16.5cm 6500i component set 270W for £49.99 sale price but I think these went like hot cakes. There are other good deals around.
16.5cm are the much better choices you will not regret fitting them. If you fitted 13cm you would always think what if.........
And the door card fits back as standard. Even with std H4 head unit the sound improvement is immense.
good luck,
Andrew
