If the witness DOES show up, you might find it beneficial to have somebody else (who knows what they are doing) represent you. I highly recommend you check this thread http://www.ontariohighwaytrafficact.com/topic7032.html and specifically the very first link in the very first post which will explain why.
Since the officer did not witness the event, he can not testify that you were the driver. They may put him on the stand and he may testify that the other witness told him that XYZ was driving and that he talked to XYZ and got the drivers license from XYZ. But a good cross-examination question that would bring reasonable doubt to this testimony is "Officer, did you see XYZ driving carelessly?" The answer should be NO.
Now the witness themselves will get on the stand, but if you are not there, how will the witness identify you? So regardless of what the witness says, a good cross examination question that would bring reasonable doubt to the witness testimony is "Can you identify XYZ in the courtroom today?" Answer is NO since you are not there.
In closing arguments you say "The officer gave no evidence that XYZ was driving carelessly as he did not witness the event. The witness could not identify that XYZ was the driver. Therefore the charge should be dropped because there is reasonable doubt as to whether XYZ actually drove carelessly".
And remember in all this you NEVER have to admit to anything, meaning you do NOT have to take the witness stand and do NOT have to testify and do NOT have to ever admit whether you did anything wrong or not. If you DO take the stand and testify then you will most likely get yourself in trouble, so sticking to finding reasonable doubt in everybody else's testimony is the better option. Read this as well:
http://www.ontariohighwaytrafficact.com/topic7039.html