(Note: Michael posted this to the Yahoo! 206 club in May '01 and is reproduced with his permission, thanks Michael!)
(Usual Disclaimer, do this entirely at your own risk!)
Very simple job, should take you about 20mins if the speakers you are buying are the same depth replacement speakers and can fit in without cutting (many should, i know for definate that the Sony 140W 16.5cm speakers (model XS-A1724) fit, and any in that range should).
You will need:
1 x Flat bladed screwdriver
"Star" screwdriver bits
1 x Snips
1 x Wire Strippers (although i manage with teeth when ones not available :-)
4 x Wire connectors (you know, screw-down plastic encased to join wires together)
Plastic wire ties
1 x Insulation Tape
Depending on the speakers you may also need a crimping tool (which usually includes snips and wire-stripper) and some length of speaker wire.
Start with the passenger side first.
There is a plastic grille infront of the speakers in the front door, this pops off if you insert the flat bladed screwdriver at the corners and prise it away (it's advisable to wrap the end of the screwdriver in insulation tape so you don't scratch the plastic).
You will see the speaker held in by four star screws. Unscrew these and take the speaker out. Unplug the line from the back of the speaker and remove it.
Write down which colour wires were paired with each other out of the four, and with the snips, cut off the connection plug. You will be left with the four wires (two come from the head unit, the other two go upto the tweaters in the door mirror housing). Strip the ends of them and twist them into the pairs they were in, basically turning them into just two wires.
Put these twisted ends into the wire connector and screw down. Into the other and of the connector, insert the wires that came with your speaker to plug into it. Use the white wire on the car's speaker wires as the "+" terminal when connecting the speaker wire in. Screw down.
If your speakers came with crimp plugs, use the crimping tool to clamp them onto the other end of the speaker wire, after stripping it a little.
Plug the other end of the speaker wire into your new speaker, using the wire ties secure any loose wire to the back of the speaker hole in the door (to prevent rattling) and then insert the new speaker. Screw it back in, pop the grille back on and you're done!
Now do the driver side!
I have heard that on some 206's, the plastic grille is a smooth part of the door with no edge and and doesn't pop-off. If this is the case, locate the two door panel bolts: one is behind the mirror casing (just pull that off) and the other on the inside (front) edge of the door, 2/3 way up. Unbolt these and then unscrew the star-screw in the door hold (the bit you grab to shut the door). Then just pull on the door panel from the edges and it will pop off (it will need good pull, but it is only held in by plastic pop-nuts) to get access to the speaker.
The rear speakers are a slightly more complicated process, let me know if you want instructions on this!
Need anymore help, let me know.