This is a step by step instruction to modify the 8043 to fit a technic minifig and a seat.

The picture "Additional parts" tell what parts needs to be added to make this mod, while "Removed parts" shows the parts that needs to be removed.

To execute this mod you only need to do a minimum of deconstruction. First, remove the Side plates on the cabin side, an release the top of the LA cilynder closest to the cabin. It is also a good idea to remove the flex axels that symbolizes the sides of the cabin, and install them last as seen in the pics marked "finished". 

Then you need to deconstruct parts of the cabin to switch two liftarms. These are shown in "Step_00". They should be replaced be liftarms two studs longer, but mounted so that they increase the over all length of the cabin by one stud.

Then you need to remove the oridginal seat, but tae care as you will be needing some of the pieces like the "blue axel/pin", the "black 2 stud 1/2 liftarm" and the "4 axel with stop".

Follow steps 1 to 4 to test build the part you are going to mount, then mount the part from step 1 on the LA cylinder side of the cabin. You will need to insert the black pin first while holding up the cylinder, then add the 3 stud long yellow liftarm before lowering it again (View 5_1). Then you are free to mount the rest of this part (View 5_2). It should look like step 5 when you are done.

The next step is simple. Just take the part from step 3 and insert it where the old part to hold the seet was inserted. Se Step_6 picture.

Step 7 consists of inserting the seat with a minifig. The feet may be difficult, but there is enough room.

In step 8 you replace the flex axles with a blue axel/pin to make them 1 stud longer. And, you are done!