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You have written that you have 14.5 volts just idling (that's good), and 14.1 volts with the headlights, fan and wipers (that's good, too) -- all assuming that's when you first start up the car when it's cold. So you will have to ask yourself, what is different about the state of the car when you're sitting and idling, and taking measurements of the battery voltage, compared to when you're driving for 10-15 miles after which you find the battery has run down. By the way, you didn't say how low the voltage dropped after that time.
I should point out at this early point in our discussion that it is perfectly normal(!), even necessary, for the battery voltage to drop somewhat -- down to, say, 13 volts or so depending on your climate and the season -- after driving around a while. This is because the charging voltage is temperature dependent: when the battery is cold, the charging voltage is higher -- if you were in Buffalo, NY, right now (viz., polar vortex), your charging voltage should be nearly 15 volts when you first start up. And when warmed up (in the then toasty engine compartment) the charging voltage must be less in order to avoid overcharging and boiling the acid solution in the battery -- less than 13 volts would be normal driving in Arizona in the summertime.
I'll assume, however, that your concern, writing this post, is because your battery is draining to far lower voltages, under 12 volts, right? If so, you may have some component that's taking a big drain from your battery...
To start, when checking for a drain, measuring volts is of little value -- it just measures the state of the battery, and with the engine running, the alternator overwhelms and masks any sign of an abnormal drain. So while voltage is of little value (largely determining only whether the charging system is working), what you really need to do determine whether or not large amounts of energy is being drained somewhere. By the way, the test light is also of little value except to determine whether a wire is live or not; I don't know what you tried to measure with the test light connected to the negative cable, since it shouldn't light up (being grounded at both ends).
You need to, instead, set your meter to amps to measure current when you're trying to find a drain. With the engine TURNED OFF (!), you set your meter, measuring amps, on the higher scales (you can work down to lower scales when you made preliminary measurements) with probes between the negative cable clamp and the negative battery post: there should be (assuming only OEM accessories, meaning no aftermarket radio, etc.) only be about 12 milliamps (for the radio presets and ECU memory), with brief pulses to 25 ma for the clock stepper motor. Anything significantly more than that, and you may have a noteworthy drain.
Report back, and we'll go from there.
Good luck.
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