Your Sergeant had no right to make this unethical request of you or to place you in this position. I can understand the position you are in but it is not okay to lie.
The right thing to do is to tell the truth. I would recommend that you write your statement in a private location where you can think clearly and then ask to see the Platoon Sergeant, 1SG, and Commander so that you can explain what happened before you turn your statement in. This may be a difficult position for the chain of command because you did not explain this to them when the Squad Leader initially stated that you wrecked the vehicle.
Odds are if you take your time and explain the situation clearly the command will work something out that is fair and just. I would also suggest you request to be transferred to another squad.
This is not an easy situation and while there is only one answer with regard to you telling the truth, there is no easy solution for the chain of command unless you have proof he was driving. With that said, I believe the chain of command will work through the situation and come to a realistic answer.
If this individual asked you to lie he has probably done something in the past that will lead the chain of command to question his integrity or actions. Relax, be professional, and be truthful.
If the command does in some way find you responsible request to see the IG or JAG for further guidance. Be advised: lying on the written accident report could be considered a false official statement or fraud and is potentially punishable under the UCMJ.