Ah, well remember the Licker that impaled Gerard's leg? Well what if it infected Gerard with the T-Virus due to them being created by the T-Virus. However Gerard was infected with an odd strain of the virus. This caused Gerard to gain slightly increased endurance and speed (nowhere near as intense as Jack's or Wesker's.) This small boost of Endurance and speed will allow him to survive the exploding car with some ease. He would be injured badly though, but he does have a few new abilities like slightly increased reflexes, a small healing factor, and slight increase in speed and endurance.
Intense heat would damage Wesker and could kill him (as well as mini Nukes), but due to such an advanced mutation he would be highly resistant still (however he still will be injured.)
Much like the Ndesu 2 after enough damage is sustained some of the black leech like tendrils on Wesker's body will unfurl and reveal an orange pulsating heart like object on his chest and back. Destroy these two and you would successfully kill Wesker.
You know the London arc truly is almost over. Heck by tomorrow it should be done.