It looks like the fire is concentrated on the driver's side of the dash there... by the headlight switch and vent. Whenever there is a short or fire it will be worse at the source. So if this is the worst area of damage... that's where I would start looking. It may be hard... but see if you can trace where the short/fire started. If it was factory wiring that caused it... it will be hard to prove as most of the evidence will be melted or burned. I've never heard of an S10 doing this... I can say one thing from experience as a car audio installer... I've seen many many people come into my shop with stereo systems and aftermarket neons, lights, etc installed by themselves and there are power wires running all over the place (unfused and definately not safe) which could easily lead to a short or electrical fire. If you have anything aftermarket in the area where the fire is most concentrated you will have a hard time getting anything for it.
Sorry to hear about your truck but I hope this helps,
Nick