I know how to download Grab Taxi for use on my Andriod phone, but I'm not sure how to fund it - or - do the drivers accept cash?

Yes to cash.

Go to Play and search "grab". Select install and follow the setup instructions.

Can either use cash or card. I set my American Express card for payments and it works perfectly in Thailand and Cambodia.

 

Always pay cash

1 hour ago, FritsSikkink said:

Always pay cash

Will the driver always have change? 

 

1 hour ago, soi3eddie said:

Will the driver always have change? 

 

No

6 hours ago, soi3eddie said:

Will the driver always have change? 

 

I always have small notes on me and give a tip, so no need for change.

1 hour ago, FritsSikkink said:

I always have small notes on me and give a tip, so no need for change.

Good practice but not always possible or convenient. You may have a few 20s, a 500 or two and some 1,000s. Your fare comes to 100 Baht and the driver has insufficient change. Then what? Nice big tip? It's for this reason I select a western credit (not debit) card as payment. Also. if any dispute simple to raise a chargeback.

 

11 hours ago, soi3eddie said:

Will the driver always have change? 

 

That’s the same for any taxi you take…  they may or may not, so it’s best to have change if paying cash. 
 

Or, if you have a Thai credit card, add that. 
 

I’m not sure if regular Thai bank account cards are accepted in these apps - worth a try…