'Gone Fishing' (Ray)
It turns out the witness from 'Silence the Sneak' isn't quite dead yet. You need to track down his ambulance and run him over.
Follow the marker to the ambulance. Once you ram it the police will be onto you. Ram the back doors until the witness falls out. The plaster body cast makes him immune to bullets and flames, so you'll need to run him over. The pursuing police cars can actually help here. Once he's truly dead the mission is complete.
Be advised that destroying the ambulance directly won't work. You'll be told it was a decoy, and a new ambulance will spawn near the stadium. In fact, in a Rhino you can continually run over ambulances as you make your way to the spawn point where you can park underneath and watch an endless succession of fresh decoys fall from the sky as you destroy them.
New Missions Available
'Marked Man' (Ray)