It would be very difficult and time consuming to rely on public transport to travel in this area, though possible. The car is undoubtedly the best choice.
Riding a bicycle is another possibility. The traffic is not too heavy on the roads around Motovun, but the road is steep to get into the city (hill top city!). However, one great advantage of a bicycle is to avoid any parking problem.
OK, there are 2 ways to get to Motovun (unless coming from the North), and they are
1.) via VIZINADA - this is the 'easy' way. The road is newer, the cars much faster, it is scenic except you will not get a view of Motovun, and it is a little bit longer but probably a little bit faster as well, so if you have a nice car and want to keep it that way or you just want to get there fast - follow the signs to VIZINADA and take on from there.
2.) via VISNJAN - this is the road most locals take when going to Motovun. It is the shorter way...but, with many many holes in the road, almost no passing anywhere, 25% hills and 120' turns, BUT - you will get the nicest, most beautiful view of Motovun coming down from Visnjan!!! And, of course, you can make it a fast ride this way as well...if you dare to have this much fun!!!
So if you like to take pictures take the Visnjan way, if you just want to get there fast - take the Vizinada way....
And yes, there is a bus connection to Motovun from Istarske Toplice.