well, after experiencing peeing problems with felix, i can tell you what i've learned:
cats do two kinds of peeing: spraying to mark their territory (standing upright, wiggle their butt and aim at the wall) or inappropriate urination (squat and make a puddle). the former is territorial and the latter is because they have a medical problem (urinary infection etc), they don't like their own litter box, or they just developed a bad habit of enjoying peeing in your garage. if you go to the feliway website (i think www.feliway.com) they have some info about the general problem and their product (expensive spray bottle of cat pheromone that sets them at ease and supposedly works very well). also a general search on cat behavior will yield many additional sites. maybe you could talk to your neighbor about it. and there's always the possibility to keep your garage closed. good luck!