Is this your blower? If so, there are two screws holding the top on. They are on top between the gear case and the fan case. After you remove them the top should slide back. There is a large screw inside the case that engages a slot cast into the inside of the top at the rear away from the fan. It holds the back down.
If it's not loose enough to slide off after you remove the screws, I'd advise a good soak in diesel. Then carefuly using a putty knife slowly tap it into the split all the way around. You're not using the putty to get the top off merely trying to break the gunk seal. Then carefully use a flathead srewdriver to pry the top toward the rear. DO NOT TRY TO PRY UP you'll break the top.
Still won't come loose? Soak some more and do it again.
Still won't come loose? Most likely the diesel is not getting to the gunk buildup on the internal holddown screw. Very carefully warm the rear of the top just past the "too hot to touch stage" and then try to pry it back.