ADDING ROOF-RAILS TO A C3 PICASSO WHICH DOESN'T ALREADY HAVE THEM
I'm no expert or even a good mechanic. Please take everything said here with a large pinch of salt and do your own research. . I tried to do inline images within tags but nothing worked so I've pasted in Google Drive links instead. If anyone knows a fix then I'll gladly edit.
1) Consult eBay / scrap dealers for a set of bolts, nuts, black plastic 'gutter' trim and the rails themselves. the first eBay seller responded to me wanting £200 but someone else offered everything for £75. You can't just buy the rails themselves, you'll need the black trim as it has cutouts for the rails to slot into and the fixing bolts are longer when rails are fitted.
2) Carefully remove the existing black plastic 'gutter trim' from the car roof by inserting a screwdriver under the front section near the driver / passenger door and lifting up. It un-clips fairly easily. Do this on both sides.
3) You should now see eight T20 Torx bolt (it is French, after all), four on each side.
WARNING (1): These bolts hold up brackets which are inside the car and can't be seen. If you take out both bolts from each pairing then the chances are that the bracket inside the car will fall away and you're scr... in trouble. Take one bolt out, replace it with the longer bolt you acquired from the scrap vehicle, then take the second bolt out and replace that too and work your way around the car. That way you'll always have something holding the brackets up while you swap-out the bolts.
The bolts you're removing are done up VERY tight.
The first cheapo Torx bit I tried, ended up merging itself with the bolt and I had a hell of a job then hacking the bolt about to get it out. After paying £2.50 for a proper diamond coated Torx bit (here)
all the other bolts came out ok via a combination of socket set + "screwdriver bit" adaptor.
4) Fit the black plastic "gutter trim" which clips in place by pushing it down.. Worth making sure you have it the right way around and lined up at the front or rear of the car before you start pushing down. It should be obvious by the position in the trim and the position of the bolts.
5) Lower the roof rails on to the bolts. Make sure you use the correct rail for the correct side of the car - they're labelled on the plastic parts "front left", "front right" etc.
6) Place the nuts on to the bolts and tighten. @c3car suggested they be tightened to 0.80 daN.m.
7) Clip on the plastic covers to each end of the rails and you're done.