Dragerking

What is the residual percentage for the 2014 GL350 BT? ( if so, will this differ in Canada ?)

ymmv

The residual values and percentages will be almost exactly what MB offers in their lease products. You'll get a number with a few percent. If you need a ball park, any of the lease projections will be accurate enough for cashflow and total cost of ownership projections as well as giving you a fair market overview. I'd suggest leasecompare or swapalease are probably two of the bigger sites.
I'm on my third MBUSA lease and they work out well so long as you start with zero cap reduction and about a 5-10% msrp top line discount to defray costs, then your cost to exit after say two years will be with a buck of a month of the lease payment and you'll start to gain equity in the car.
I'd suggest structuring the lease to be longer rather than shorter because of prevailing low interest rates and the rule of thumb is to have a lease end around the end of the warranty period, so four years tends to be a good break-even on $100K +/- ... as ever, caveat emptor, trust nobody ... else posts like this on the Internet. : )

Dragerking

From what I know from the Canadian Configurator:

MRSP: $85500
Residual: $51000 for 36 months and 58000 for 24 months.

So, How would I negotiate a very good lease?