Bus 166 Extra has a sign on the front windows in English indicating that is going to immigration (Government complex) and stop at the beginning of victory monument bus stop at the beginning of Phayathai road side, first lane, meanwhile the other 166 bus will stop at the end of the third lane of the same bus stop. There are two buses The air condition blue bus cost 22 Baht and the red one cost 10 Baht . If you take the buss that will leave exactly at 08.00 AM, make sure to be there few minutes before and you will get a sit.
The same bus will leave Immigration office from the same spot where you dropped, back to victory monument but you may have to wait a little bit.