I used one of those vinyl repair kits...my crack was very small, but in the same place you mentioned. The biggest thing with the kit is you have got to do the prep work..don't just clean the dash like you normally do...If I remember correctly I had to clean it, then do a final wipe down with mineral spirits...it took me about 2 1/2 hours to fix the crack...looked better than it did, but you could still tell there was a fix. I ended up dying the dash black after I fixed it, and it looks much better.
(by the way, this was a replacement dash I bought from Jim (hotstuff)...my original dash was HORRID...the speaker spots were missing etc.. so anything looks better than that one!
Brandy