This study proposes an index modulation (IM) technique for affine frequency division multiplexing (AFDM) signals and examines its communication and sensing performance toward integrated sensing and communication (ISAC) systems. The power levels of subcarriers are utilized as modulation indices while also transmitting data symbols within each subcarrier. Thus, the proposed AFDM with subcarrier power-level index modulation (AFDM-PLIM) maintains all subcarriers active at all times to achieve a higher spectral efficiency compared to other AFDM-IM techniques, where some of the subcarriers are turned off for IM. A low complexity estimator and subcarrier grouping are also proposed to reduce the computational complexity of the maximum likelihood estimator. Furthermore, this study also examines the delay and Doppler ambiguity functions of the proposed AFDM-PLIM and evaluates its range estimation performance. The results show that its sensing performance is better than AFDM-IM waveforms due to keeping all subcarriers active at all times.