Why do the wipers on your 99 solara come on by themselves?

Since I am not familiar with your 99 Solara, this answer will be generic, but probably applies to your vehicle. Most vehicles for the last 30 or so years have multiple function windshield wiper systems. Most modern wiper systems include a high speed setting [cycle rate], a medium speed setting, a low speed setting, AND an INTERMITTANT [or variable delayed] setting, which is slower than even the low speed, and in addition, the interval of delay between wipes is variable and usually is adjusted by the vehicle operator with a slide switch, or a rotary switch on the end of the wiper control lever. If you inadvertantly bump or hit the control lever or switch, the intermittant function cycle can be initiated, causing to wiper to operate, seemingly as if by magic. Or, if the control is set to the intermittant position. Find your owner/operator manual to learn how to use or disable the intermittant function of your wipers. If you do not have a manual, go to a dealership new or used car salesman and ask for instructions: they are generally very happy to share their knowledge with you, especially since all satisfied owners of their vehicles are a potential buyer later on. If this method is unsuccessful, then try one of the parts people, and then one of the service department service writers. If that should fail, then when you are at Wal-Mart, or any similar large parking lot, keep an alert watch for a vehicle like yours, and wait for the owner, and then explain your problem and ask them to explain [even demonstrate/teach] operation of their wiper system. If that fails, then to my knowledge, your last resort is to go to the Parts Department of the dealership for your vehicle. and ORDER [you gotta buy and pay for] a replacement Owner/Operator manual, making sure that it is for your specific make, model, and year.j3h.