This subject keeps coming up, so I've dug out the post I did in December
2001 (and repeated in May 2005) after I replaced a matrix on my Mark 1 J
Reg 405 turbo diesel estate, which I did only after a lot of advice and
encouragement from others in this group. And if you clamp the heater
hoses under the bonnet at the bulkhead you don't need to drain the system.

In fact apart from the one fascia screw I didn't need to touch the
fascia at all. But note this is a 1992 Mark 1. This, at the time, saved
me £440 over the book price quoted by Peugeot London.

WARNING: It worked for me, but I take no responsibility whatever for
whatever mess you might get into. Note that at the time another poster
said Mark IIs were harder and dash did have to come out. (See
immediately previous post)

And once again I point to this link (originally posted by someone else)
http://rangerovers.net/repairdetails/heateroring.html
which is for a Rangerover but the O-ring housing and construction is
virtually identical to a 405 - it helps to see the O-ring cover to know
what you are going for.

I guess it was about an hour to get the matrix out. Half an hour to fit.
And an hour and a half to reassemble. But I'm slow. All while very
uncomfortably laying with the door frame sticking in my ribs - so get a
pillow!

Mark 1 1992 405 turbo diesel estate, without air conditioning.
Basically you get the glove box out and everything else is easy.

1. Remove the soft cowl under the glove box.
2. Remove the lower side of the central tunnel. It's plastic and pops
off four or five clips.
3. Remove the lower plastic inner door surround, one screw and then pops
off.
4. Remove the eight or nine screws inside the glove box cover, and pull
off outer skin.
5. Tap out the roll pins inside the cover end of the two restraining
arms with a suitably sized drill bit, split pin or drift. The glove box
door is now free. [[Note how the springs go - I didn't and never got
them back right]]
6. Inside the glove box, with a mirror,  find one screw in the centre
top edge and one in the left top edge with heads facing away from you.
Remove.
7. On the left side near the big fascia screw, find a small screw
pointing up. Remove.
8. On the right side, near the big fascia screw, with a mirror, find a
small screw pointing up, remove.
9. Well above this, in the mirror, you can just see the last retaining
screw, which I could not reach.
10. Free the glove box, starting on the left. There is a big cable
bundle tied to the box. Cut tie.
11. At the top at the back is a clip which holds the edge. Prise edge free.
12. The only thing now holding the glove box is the last unreachable
screw noted at 9. I decided to break the plastic mounting tab rather
than pay £450. I did this by pulling the box down and toward me. I don't
think the box will be that much weaker without that screw.
13. Under bonnet remove battery and clamp heater hoses. [[ KNOW YOUR
RADIO CODE FIRST!]]
14. Back inside, take out the big fascia screw just below matrix and
near radio.
15. Undo screw between two heater pipes [[A sod little x-head screw. See
rangerover link for talk about replacing with Allen head screw]]. Push
pipes back and up and catch no more than a litre of escaping coolant.
16. Release clips holding matrix as if in a book's slip cover, and pull
it out (to the left) taking care to hold clear pipes and bit of fascia
mount near screw you removed. It just makes it.
17. Instal replacement (£56 inc Vat from Peugeot.. less from factors),
connect, top up coolant etc.""

[[ Obviously use new O-rings ... maybe they were all that were at fault!
And get the x-head screw tight. Leave glove box out until you are sure
you have fixed leak!  Reassembly is the reverse, but you may need
chewing gum or a magnetic screwdriver with some of the screws.]]

[[I found that the replacement matrix did not go in all the way, though
it works fine without it.  Subsequently a helpful posted pointed out
that sometimes there is a plastic cover off the end of the old matrix
that gets stuck in there if so you can usually hook it out(or remove the
cover off the end of yours). Subsequently to be tidy I hoiked out the
matrix and fixed this. ]]