One of the very first mods I did to my T4 was to upgrade the dash speakers. This wasn’t a permanent solution to the lack of good tunes in the van, but as a very easy and temporary fix – it’s a must.
The standard head unit and dash speakers are as rubbish as you can get, so upgrading them is essential. I’d recommend getting the best head unit you can afford. The speakers are a bit more flexible as any price 10cm / 4inch speaker is going to sound much, much better than the originals. Eventually you will want to put much bigger and better speakers than 10cms in your van, so spending a fortune on them is a false economy. I used these: Alpine SXE 1025 Car Loudspeakers which at a low £20 did the trick and were lots better than the standard van speakers!
Changing them over is a piece of cake, the grills pop off with a screwdriver and then you just have to disconnect the old speaker and reconnect the new. You can even buy little converters so you don’t need to do any wiring!