I am a veteran of the Volvo dash crack prevention repair war of 1994 to 2000. A crack will get wider w/ time so a small hole at the end will just tear. A 1/4 in hole is needed to stop the crack. Dash filler material sold in stores are only good for the top 1/8 inch and grain matching.
Drill the end(s) with 1/4 bit.
Get a dentist pick (next time you are in your dentist's torture chair ask the assistant what they do w/ old ones then steal one for your pains) and pick out the rotted dried sponge like material 1/3 inch or deeper below the crack.

The portion under the dash skin along the crack must get clean.

Fill crack w/ flexible filler (Goop works well but anything that dries flexible works). When the filler dries it will shrink.

Get a piece of vinyl (mbtex, cheap shoes, wallets..fake leather) and cut 1/2 inch longer than crack by 1/2 inch.

Without making crack bigger fit this piece under dash skin in crack (5 or 6 tears later you will become a pro).

Using the dentist pick apply flexible adhesive between dash (under the crack) and vinyl strip.

Let dry.

Use dentist pick to pick off any glue or junk that remains above dash surface at crack.

Fill crack with dash repair paste and iron on corresponding pattern.

Enjoy the perfectly repaired crack and then start repairs on all the collateral damage caused by repair.

You will also know which picks to heist at your next dental visit.

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82 300D....went to MB heaven
90 350 SDL....excercising con rods