Swansea, with just one win in their last eight league games looked to be heading for yet another defeat on their travels after trailing 2-1 at the break, despite taking the first half lead.
Northampton, chasing a play-off place, had begun brightly with Scott McGleish, Lee Williamson and Pedj Bojic all going close.
But Swansea should have taken a 21st minute lead when Paul Connor found Adrian Forbes in front of goal, but he missed the target from just six yards.
Williamson went close again for Northampton before Connor put Swansea ahead on the half-hour mark, applying a simple finish from close range after Trundle created the opening.
But Swansea's lead was short lived as Martin Smith equalized four minutes later, firing home first time after his initial corner had not been properly dealt with by the Swansea defence.
Marcus Bean went close to restoring Swansea's advantage when he ran through the Northampton defence before poking the ball wide.
New signing Andy Kirk headed Northampton into a 2-1 lead in the 40th minute on his home debut after Williamson and Joshua Low combined on the right.
Kirk and Low both went close after the restart but, in between, Northampton keeper Lee Harper produced a finger tip save to keep out a goal bound header from Izzy Iriekpen in the 63rd minute.
Substitute Andy Robinson fired over shortly afterwards as Swansea sought an equalizer which finally arrived in the 70th minute.
The impressive Connor found space on the left flank and his low cross was allowed to reach Trundle at the far post and he made no mistake to earn Swansea a share of the spoils.