Drain plug for what?
open the hood, look down between the rad. and the engine. There it is.
Only the 2.3L HSC Inline 4 will fit in the 1986 Mercury Topaz. If everything in the engine bay were gutted, perhaps something larger such as a 3.0L V6 from a 1992-1994 model MIGHT fit but it would require a complete running 92-94 Ford Tempo or Mercury Topaz with the V6 engine as a donor car (and, if you could find such a car, it would be more worth your time/money to invest in it v.s. a 1986 model). The automatic transmission from a Taurus WILL NOT WORK in a Tempo/Topaz.
Just take the covers off the steering column, switch is there.
Behind the glovebox
the fuel filter on a 1986 ford ranger is located on the inside of the frame on the drivers side in front of the gas tank.
the fuel filter is located on driverside rear in front of the rear wheel.
there isn't one
The fuel filter for a 1986 Chevy G20 should be mounted on the frame under the van. The filter is an inline type and should be easy to change.
On the fuel line going to the carburetor.