Just noticed you have a convertible. In that case it’s easiest to pull the head unit and measure the speaker paths from the wiring harness (unplugged from the HU). If you put the HU in yourself then you should have the necessary info to determine which wires go to which speakers. If not you may have to reason it out (the HU's manufacturer may have the owner's manual online with some wiring info). The wires will add a little bit of resistance to the measurement. So if the speaker has a voice coil resistance of 3.2 ohms the wiring and connector may add another .4 ohms to that number for a total of 3.6 ohms. To use the meter set it to its lowest range (either 20 ohms or 200 ohms range, I’m assuming it’s a digital meter). Numbers around 2 ohms or below could indicate a problem with that speaker’s voice coil.