What good would a "boycott " do anyways.
So if a car company has higher prices in the US than Canda you refuse to buy their car and then what?
Buy a different brand?
Fine so you won't buy a BMW because their prices are higher so you buy an Audi instead? Aren't their prices also higher in Canada than US? You have to buy SOME car and if you cross all the companies off your list that have higher prices in Canada what car CAN you buy?
Even Porsche who dropped their Canadian prices still don't match up so what do you do buy a bike?
An indefinite boycott is impractical. People have to replace their cars at some point. Not everyone can or is willing to purchase through a US dealer. 
I think the most effective way would be to pick one month to boycott. Get everyone to put off their purchase for 1 month, just enough to screw up dealer inventories and p*ss off a bunch of commission based sales people who would no-doubt give their employers an earful. Sure, they'll make up the difference the following month, but I think the disruption in sales (for both months) would cause a lot of turmoil.
So who out there has media clout? 
That would have basically zero effect.
what would you do place an ad in the papers saying "OK everyone we are encouraging everyone to not a buy a BMW this month as a protest against the price difference in the US"
The dealers will read it and all call BMW Canada saying you have to cut prices to US levels? What incentive is there for BMW to do it. They won't lose any sales they will gain them all back next month.
Dealers don't give a

if commissioned sales people complain. Commissioned sales people complain all the time. Dealers see them as an expendable commodity.
Plus a boycott is only useful if it is supported. The minute that AD calling for the boycott hits the paper all dealers have to do is run their own ad.
"We are at the mercy of the prices BMW sells us the car at. We have voiced the concerns of our valued customers to BMW. As a goodwill gesture we will spend $s out of our pocket to prove our commitment to Canadian consumers with a special $1000 cash incentive this month (and this month only) on all in stock 3 series. (mind you they will likely wiesel $1500 a car from BMW to help them trough this difficult time. BMW will gladly pay this to make it blow over rather than loses big $ from a price drop for the whole year) Then watch the customers who KNOW that BMW won't drop prices because of the "boycott' anyways flood back to the showroom to take advantage of the "deal" and watch the boycott fall apart.
Boycotts DON"T and will not work under any circumstances. Just too many people to control and no way to do it.
The only thing that could possibly have any effect is a massive migration of people buying from the US. Not a boycott. A real shift of sales (repeated over a number of months) from Canada dealers to US. And it has to be documented. If dealers in Canada see a drop in sales and know and can prove that the sales are being lost to the US then they have a legit complaint and a strong enough voice to get action. They have an investment and franchise agreement with the Car maker that the maker the dealers provide the service support network needed for the manufacturer to operate in Canada. Only if this is put in jeopardy will there be any action taken.
even then its not at all likely that the action will be a price drop. Its more likely that they will "protect" their Canadian dealer by discouraging cross boarder shopping. They don't want people buying in the US. they want them buying in Canada at a higher price. So it makes more sense to force people to buy in Canada than it does to drop prices and make less money.