I tried the search, but surprisingly few people have had documented latch issues. This thread and
feebleknievel's thread helped me get started as well.
I had to open up the Downtown 300i yesterday because the seat latches would not release when the key is turned at the ignition switch.
edit- it may be possible to open the front of the bike first and play with the cable adjuster at the ignition. Turn the adjuster so the key becomes rather difficult to turn, and then give the key a solid turn. With some luck, there will be the rear seat will pop open. This will avoid prying.
At the rear of the bike, the big grab rail/luggage support comes out, but it can be difficult to get an Allen key into the bolts on the sides of the bike. A pair of pliers helped grab the Allen key and give me leverage. Next, the plastic middle panel above the taillights comes out. There are two Philips screws, but also two plastic tabs that securely lock the middle panel to the side panels. Use some tape on a screwdriver and be very careful to reduce scratching or chipping of the paint. If in doubt, use a flashlight to examine where everything is and take your time. The panel may come out with some force as it's under quite a bit of tension while prying.
The latches should then be in sight. Use a flat blade screwdriver to push the latch mechanism on the left side, and use your fingers to pull the latch on the right side. The seat should audibly pop loose.
For me, the right side cable had a little tension when I turn the key, but not enough to unlatch. The left side cable had absolutely no tension. I could adjust the right side cable to a usable range with the adjusters under the seat, but the left side cable adjuster I could max tighten but feel no tug from the key side.
If the adjusters under the seat work fine, then you can stop here. Be sure to manually test the latch mechanisms and key release a dozen times to make sure they latch and unlatch properly before putting the seat back on! Sometimes the latches will unlatch themselves if you have the cable too tight. Too loose and the latch feels sloppy. If you run out of adjustment, then keep reading.
I then removed the front of the bike to access the ignition switch. You can then see the cable assembly as it runs through the bike. In the Downtown, the ignition pulls on an upper cable that runs down by the coolant reserve bottle where it splits into the two cables that run to the seat latches at the rear. It is all one piece, as the splitter is fully enclosed and doesn't look serviceable.
The barrel adjuster at the upper cable controls how far the upper cable and thus seat cables are pulled when you turn the key. I had to loosen the adjuster to increase the slack on the cable, allowing the cable to be pulled a longer distance. You know when it's too tight when you cannot turn the key very much or at all. Increasing the slack gave my left seat cable a noticeable pull again. Test everything and remember to secure the locknut on the upper cable adjuster before you put the front plastics back on the Downtown.
