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.
Walkthrough
Enter the toilets at Belleville Park. Machowski explains the situtation. 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.
