Generally a fair bit of maths is involved, through my mechanical engineering course I did three purely maths subjects that can get a bit tricky. After that my experience is that you pretty much just need to understand how to apply and manipulate equations so i guess that's algebraic stuff. If you do engineering then you will work with equations or maths relationships in some form almost every day (unless you get into management) but if you know or are happy to learn to work with geeky looking equations then you should have no problem.
A bridging course is up to you. One text book that I bought as part of a maths course started with the basics anyway, it even taught addition and subtraction. So if maths isn't your strong suit you could probably pick it up with some hard work.
The light vehicle trade cert. may or may not help depending on the industry you are applying to work in. It may not necessarily be useful if you worked at a power station for example. But saying that it might be a good idea to stick with your apprenticeship and finish that first. It would look good on a resume if you have seen it through to the end and shown that you have an practical interest with nuts and bolts and how things work. Also as an engineer who also has a trade you would probably have less issues working alongside tradespeople than some engineers.
If you were looking to enter into the motor industry as an mechanical engineer then the trade would probably be an excellent thing to add to your resume.