BTDT. Not a fun job. You need the right tool. Picture a screwdriver, with a can-opener screwed onto the end of it, with a hole in the middle of the can-opener.... (I'll see if I can find a pic of one.)
Put the glass into the main piece of the seal, then put the seal onto/into the opening. Then you use the tool to spread open the channel in the seal, then feed the locking strip into the channel through the hole in the tool. The straights aren't bad; but when you get to the curves, it's hard to keep the tool moving onwards....
And..... good luck!
-L
'72 SIII SW 88"
'60 SII 88" RHD