Is it possible to ascertain the current whereabouts of a car which
used to belong to my late brother? The car is a 1986 Mercedes Benz 180
 D180 TJR

usually the Tag number will not do you much good on a car that old
since it may be sold out of state , been in a car wreck and totalled
and then patched up and sold

you could try several services that exist on the internet (none of
them are free) but you would be better off if you had the VIN number
of the car

just google for "Car Tag number" and you will get a plethora of sites
that offer information for a price (usually in the $30 range)